1. Field of the Invention
The present invention relates to a sample observing apparatus and a sample observing method for observing an appearance of surface of a sample and more specifically to an apparatus and a method for observing the appearance of surface of the sample by irradiating an electron beam thereto.
2. Description of Related Art
Conventionally, there is a scanning electron microscope for example as an apparatus for observing an appearance of surface of a sample. The conventional scanning electron microscope enables one to observe the appearance of surface of the sample by detecting secondary electrons produced when a beam of electrons is irradiated to the surface of the sample. At this time, acceleration voltage is applied to the electron beam produced by an electron gun in order to irradiate the beam to the surface of the sample. For instance, the electron beam is irradiated to the surface of the sample by setting electric potential of the surface of the sample at reference potential such as earth potential and by applying negative voltage to the electron gun. An apparatus for detecting the secondary electrons is disposed at position apart form the surface of the sample by an adequate distance.
The apparatus having the configuration as described above may acquire a secondary electron image showing the appearance of surface of the sample by scanning the surface of the sample by the electron beam. By the way, the electron beam needs to be focused on the surface of the sample in order to observe the surface of the sample with precision. To that end, the scanning electron microscope is provided with lenses for converging the electron beam.
However, the conventional apparatus has been unable to acquire the appearance of surface of the sample with precision when the surface of the sample is charged with electricity, thus causing potential different from the reference potential. When the surface of the sample is charged with positive electric charge for example, the produced secondary electrons may be attracted to the surface of the sample and may not be able to reach to the apparatus for detecting the secondary electrons. Therefore, it is unable to detect the secondary electrons with precision and is difficult to acquire the secondary electron image with precision accordingly.
Still more, when the surface of the sample is charged with electricity, the acceleration voltage of the electron beam may vary, thus varying the level of focus of the electron beam on the surface of the sample. Because the divergence of the electron beam on the surface of the sample depends on the level of focus of the electron beam, it is difficult to acquire the secondary electron image with precision also in this case.
It is therefore an object of the invention to provide a sample observing apparatus and a sample observing method that can solve the above-mentioned problems. This object may be achieved by combining features described in independent claims of the invention. Dependent claims specify further preferable embodiments of the invention.